Magnetic field therapy uses magnets to maintain health and treat illness. The human body and the earth naturally produce electric and magnetic fields. Electromagnetic fields can also be technologically produced, as with radio and television currents. Interactions between the body, the earth, and other electromagnetic fields are believed to cause physical and emotional changes in humans. Magnetic therapy is used for a wide range of health problems, including joint problems (such as arthritis), migraine headaches, pain (including mild to moderate pain after surgery and long-term pain), depression, cancer, and overstretched muscles or injuries to muscles, ligaments, and tendons. It is believed that magnetic therapy induces electrical fields within the bio-molecular structure and stimulates cells to correct abnormalities. However, many devices used in magnetic therapy are large and unwieldy, do not create an even or uniform magnetic field in the subject, are too complex for a lay person to use, or are subject to damage when exposed to water.